Networking and Internetworking - Guide to Cloud Computing for Business and Technology Managers - page 50

ensure that the pieces all arrive correctly at the other end. Furthermore, all
this must be done efficiently and in a way that isolates the upper layers from
the inevitable changes in the hardware technology over the course of time.
The transport layer also determines what type of service to provide to the
session layer and, ultimately, to the users of the network. The most popu-
lar type of transport connection is an error-free point-to-point channel that
delivers messages or bytes in the order in which they were sent. However,
other possible kinds of transport service exist, such as the transporting of
isolated messages with no guarantee about the order of delivery and the
broadcasting of messages to multiple destinations. The type of service is
determined when the connection is established.
As an aside, an error-free channel is completely impossible to
achieve; what people really mean by this term is that the error
rate is low enough to ignore in practice.
The transport layer is a true end-to-end layer; it carries data all the way
from the source to the destination. In other words, a program on the source
machine carries on a conversation with a similar program on the destination
machine, using the message headers and control messages. In the lower lay-
ers, each protocols is between a machine and its immediate neighbors, and
not between the ultimate source and destination machines, which may be
separated by many routers. The difference between layers 1-3, which are
chained, and layers 4-7, which are end-to-end, is illustrated in Figure 2.1.
2.2.3.1.5 Session Layer
The session layer allows users on different machines to establish sessions
between them. Sessions offer various services, including dialog control
(keeping track of whose turn it is to transmit), token management (prevent-
ing two parties from attempting the same critical operation simultaneously),
and synchronization (checkpointing long transmissions to allow them to pick
up from where they left off in the event of a crash and subsequent recovery).
2.2.3.1.6 Presentation Layer
Unlike the lower layers, which are mostly concerned with moving bits
around, the presentation layer is concerned with the syntax and semantics
of the information transmitted. In order to make it possible for computers
with different internal data representations to communicate, the data struc-
tures to be exchanged can be defined in an abstract way, along with a stan-
dard encoding to be used on the wire . The presentation layer manages these
abstract data structures and allows higher-level data structures (e.g., bank-
ing records) to be defined and exchanged.